Conventionally employed is an electric power steering apparatus for applying steering assist force to a steering mechanism by driving an electric motor, such as a brushless motor, in accordance with steering torque imparted to a steering wheel (handle) by a driver.
A permanent-magnet motor, which includes a stator around which a coil is wound and a rotor provided with a permanent magnet, are used as a brushless motor incorporated in the conventional electric power steering apparatus in many cases. The permanent-magnet motor may be suffered from cogging torque. The cogging torque is caused by a magnetic attraction force between a core of the stator and a magnetic pole (magnet) of the rotor in the state where electric current is not supplied to the coil of the stator.
The foregoing cogging torque causes deterioration of steering feeling in the electric power steering apparatus. Accordingly, various attempts have been performed to reduce magnitude of the cogging torque. For example, it is known that the cogging torque is reduced by appropriately defining the number of the poles of the permanent magnets and the number of poles (teeth) provided in the stator. The number of the poles (teeth) in the stator is also referred to as slot number because this number is same as the number of spaces (slots) interposed between two adjacent poles (teeth).
An 8poles-9slots type electric motor, which includes eight poles in a rotor and nine slots in a stator, is incorporated in an electric power steering apparatus. Such power steering apparatus is described in non-examined Patent Publication JP 2001-275325, for example. A 10poles-12slots type electric motor, which includes ten poles in a rotor and twelve slots in a stator, is also widely incorporated in electric power steering apparatuses. These electric motors can be manufactured at relatively low cost and be small in size thereby being applicable to the electric power steering apparatuses.
However, further reduction of the cogging torque is anticipated to improve the steering feeling in the power steering apparatus. In order to achieve the further reduction of the cogging torque, it is required to change the structure and scale of the poles (teeth). In case that the width of each of poles (teeth) is set to extremely small, reduction of the cogging torque is not expected, because the width of each of the poles corresponds to width of magnetic path through which magnetic flux from the pole of permanent magnet passes.